1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls including a center, an envelope layer, and a cover.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a long flight distance. Flight performance also correlates with a spin rate. Flight at a low spin rate results in a proper trajectory, thereby achieving a long flight distance. In light of flight performance, a golf ball with high resilience performance and to which spin is difficult to impart, is desired. Golf players also place importance on feel at impact of golf balls. Golf players prefer soft feel at impact.
There is a golf ball having an outer-hard/inner-soft structure. The launch angle of the golf ball is high when the golf ball is hit with a golf club. The spin rate of the golf ball is low when the golf ball is hit with a golf club. The high launch angle and the low spin rate can achieve a long flight distance.
A golf ball having a flexible center can have an outer-hard/inner-soft structure. Use of a cover with a high rigidity can also achieve the outer-hard/inner-soft structure. Use of an ionomer resin having a high acid content or an ionomer resin having a high degree of neutralization for a cover can achieve a cover with a high rigidity.
There is a golf ball having a hollow core. The golf ball has an ultimate inner-soft structure. The moment of inertia of the golf ball is great. The ultimate inner-soft structure and the great moment of inertia can suppress spin of the golf ball.
Use of a cover with a high rigidity can also achieve an outer-hard/inner-soft structure. Use of an ionomer resin having a high acid content or an ionomer resin having a high degree of neutralization for a cover can achieve a cover with a high rigidity.
U.S. Pat. No. 5,688,869 (JP-H06-80718) discloses a resin composition used for a cover of a golf ball. The resin composition includes an ionomer resin having a high acid content.
U.S. Pat. No. 6,042,488 (JP-H09-10357) discloses a golf ball that includes a core, an inner cover, and an outer cover. The outer cover is formed from a resin composition. The resin composition includes an ionomer resin having a high acid content.
U.S. Pat. No. 5,725,442 (JP-H09-56848) discloses a golf ball that includes a core, a mid layer, and a cover. The core has a two-layer structure. The principal component of the mid layer is a polyester, and the principal component of the cover is an ionomer resin. The hardness of the cover is great.
U.S. Pat. No. 5,977,264 (JP-H10-127822) discloses a golf ball that includes a core, a mid layer, and a cover. The cover is formed from a resin composition. The base material of the resin composition is an ionomer resin that includes a diamine complex.
US2003/008975 (JP-H10-314341) discloses a resin composition used for a cover of a golf ball. The resin composition includes an ionomer resin and a rubber-like functionalized copolymer.
U.S. Pat. No. 6,182,970 (JP-H11-128399) discloses a golf ball that includes a core and a cover. The core is hollow.
WO98/40127 (JP-2001-514561) discloses a golf ball that includes a core, a mid layer, and a cover. The cover is formed from a resin composition. The resin composition includes an ionomer resin and a polyamide.
US2006/293121 (JP-2007-622) discloses a resin composition used for a cover of a golf ball. The resin composition includes an ionomer resin, a thermoplastic resin, and a thermoplastic resin having an acid group.
In a golf ball having an outer-hard/inner-soft structure due to a flexible center, the hardness distribution from the surface of a mid layer to the central point of the center has a large step at the boundary between the center and the mid layer. The step impairs suppression of spin. The flexible center impairs resilience.
In a conventional golf ball having a hollow structure, a layer that surrounds an inner space is hard. The hardness at the inner space is zero. In the golf ball, the hardness difference between the inner space and the part that surrounds the inner space is great. When the golf ball is hit, the part that surrounds the inner space deforms greatly. Excessive spin occurs because the recoil (torsional return) of the part is small. The hollow part cannot contribute to resilience performance. The flight performance of the golf ball is insufficient.
In a golf ball having an outer-hard/inner-soft structure due to a cover with a high rigidity, the cover impairs feel at impact. A cover with a high rigidity due to an ionomer resin with a high acid content has inferior durability. A cover having a high rigidity due to an ionomer resin with a high degree of neutralization has inferior moldability.
Golf players' requirements for golf balls have been escalated more than ever. An objective of the present invention is to provide a golf ball having excellent flight performance and excellent durability.